Character discrimination using spatial filtering for defect detection in aperiodic patterns

Previous application of spatial filtering to integrated circuit photomask inspection used intensity filtering and took advantage of the grating effect of the periodic character of the photomask features. A stop filter of 0.8 mil diameter spots in a rectangular array with a spacing of 1.6 mils corresponding to the periodic distance in the mask was placed in the diffraction or Fourier transform plane. Aperiodic features were then detected.